Spectroscopy is a well-established analytic tool for a vast number of applications, such as inspection of food items, etc. Spectroscopy in the infrared range is particularly relevant, due to the distinct molecular vibration bands found in this range. However, low-noise detection of infrared radiation is challenging due to the thermal background radiation. The most widely used infrared spectrometer is the Fourier Transform Infrared (FTIR) spectrometer.
Common FTIR spectrometers must scan a reference mirror with very high precision on a centimeter scale, requiring an extremely high precision mechanical system, with associated high costs and generally a low tolerance for vibrations. Furthermore, detection of the radiation is commonly performed with cryogenically cooled detectors—adding both operating cost and complexity.
A mid-IR spectrometer based on frequency up-conversion was described in “High-resolution mid-IR spectrometer based on frequency upconversion”, Qi Hu, et al., Optics Letters, 37(24), pp. 5232-5234, 2012. A wavelength range from about 2.89 μm-3.00 μm was up-converted using three different temperatures of the nonlinear crystal to phase-match different wavelength ranges. A drawback of temperature tuning the nonlinear crystal is that slow temperature changes must be used to avoid damage to coatings on end faces of the nonlinear crystal, which is otherwise a problem due to different thermal expansion coefficients of crystal and coating. Thus, acquisition time of a full spectrum in the described spectrometer will be on the order of minutes. Furthermore, a relatively narrow wavelength range was detected.
Hence, an improved infrared spectrometer would be advantageous, and in particular an infrared spectrometer having relaxed mechanical tolerances and/or less stringent requirements on cooling would be advantageous.